In various industrial processes, water is widely used as a medium in which materials are temporarily suspended or dissolved. Under processing conditions such as, for example, harsh agitation, mechanical shear, filtering or boiling, certain aqueous systems can entrain air, which results in a foaming of the aqueous medium. Foaming decreases processing efficiency and/or can yield unacceptable products.
In the industrial manufacture or processing of foods, for example, considerable significance is attached to the control and prevention of foaming. For instance, in the industrial processing of sugar-containing plant juices (which is practiced on a large scale in the production of sugar from sugar beets), certain difficulties are caused by excessive foaming at the juice recovery and purification stage. Excessive foaming also occurs during the manufacture of potato products. Defoamers used in the food industry must, of course, be physiologically safe. Additionally, defoaming agents used in the potato-processing industry must be capable of regulating starch foam.
Dispersions of solid particles in water insoluble organic liquids have been widely used for controlling foam in aqueous systems. Such dispersions usually contain surface active agents which facilitate the spread of the dispersions to the interface of the aqueous system.
Defoamers have been used as surface active agents for many years. Specifically, defoamers which are based on polyoxyalkylene copolymers (for example, triblock copolymers of ethylene oxide (“EO”), propylene oxide (“PO”) and EO or of EO, butylene oxide (“BO”) and PO) are known. In general, these defoamers have both hydrophobic and hydrophilic blocks. At increased temperatures, these defoamers are insoluble in solution, thereby causing an increase of the surface tension of the system, which results in foam collapse.
The “cloud point” is a well-known term which refers to the temperature at which the defoamer becomes insoluble in solution. At this temperature, a second phase is observed (i.e, the solution becomes cloudy). Thus, at temperatures above the cloud point for a given system, the defoamer acts as an insoluble surfactant, thereby displaying “defoaming” properties. At temperatures below the cloud point, however, the defoamer becomes soluble in solution, thereby reducing the surface tension of the system.
As mentioned above, defoamers for aqueous systems are known. See, for example, U.S. Pat. Nos. 6,387,962 and 6,057,375. Specifically, defoamers which are prepared in the presence of a basic catalyst (such as potassium hydroxide (KOH)) are known. See, for example, U.S. Pat. No. 6,057,375.
UCON 50-HB-5100, which is commercially available from Dow Chemical Company, is an example of a high molecular weight defoamer which is based on polyoxyalkylene copolymers prepared in the presence of KOH. Specifically, UCON 50-HB-5100 is a butyl ether of an EO/PO glycol which contains about 50 wt. %, based on the total weight of the butyl ether, of an EO cap. UCON 50-HB-5100 has a hydroxyl equivalent molecular weight of about 3930 Da, an average hydroxyl number of about 16 mgKOH/g and a viscosity of about 1100 cSt at 40° C. UCON 50-HB-5100, however, has a high cloud point, i.e., a cloud point (1% aqueous solution) of about 50° C.
Another known defoamer which is prepared in the presence of KOH is PLURONIC L-61. PLURONIC L-61, which is commercially available from BASF Corporation, is an example of a low hydroxyl equivalent molecular weight defoamer which is based on polyoxyalkylene copolymers prepared in the presence of KOH. Specifically, PLURONIC L-61 is a butyl ether of an EO/PO glycol which does not contain any internal EO but which contains about 13 wt. %, based on the total weight of the butyl ether, of an EO cap. PLURONIC L-61 has a hydroxyl equivalent molecular weight of about 1000 Da, an average hydroxyl number of about 56 mgKOH/g and a cloud point (1% aqueous) of about 24° C. While PLURONIC L-61 does have a low cloud point, it also has a low hydroxyl equivalent molecular weight.
Known defoamers which are based on polyoxyalkylene copolymers (such as UCON 50-HB-5100 and PLURONIC L-61), are prepared by oxyalkylating a low molecular weight starter compound (such as propylene glycol, glycerin or butanol) with PO and/or EO in the presence of a basic catalyst (such as KOH) to form a polyoxyalkylene defoamer.
In base-catalyzed oxyalkylation reactions, PO and certain other alkylene oxides are subject to a competing internal rearrangement which generates unsaturated alcohols. For example, when KOH is used to catalyze an oxyalkylation reaction using PO, the resulting product will contain allyl alcohol-initiated, monofunctional impurities. As the molecular weight of the polyol increases, the isomerization reaction becomes more prevalent. As a result, poly(propylene oxide) products having a hydroxyl equivalent molecular weight of 800 Da or higher prepared using KOH tend to have significant quantities of monofunctional impurities.
It is known, however, that when PO is used for addition polymerization in the presence of KOH, a monol having an unsaturated group at the terminal chain is increasingly produced as a by-product as the polyoxypropylene polyol increases in molecular weight. The practical result of this is that it is very difficult to prepare polyoxypropylene polyols having a hydroxyl equivalent molecular weight greater than 3000 in an anionic polymerization reaction catalyzed with KOH.
For these and other reasons, polyoxypropylene polyols are often capped with EO groups. For example, a polyoxypropylene triol having a molecular weight of about 4200 Da, prepared in the presence of KOH, can be capped with EO by adding EO rather than PO during the last stage of oxyalkylation. Adding EO to produce a triol having a molecular weight of about 6000 Da (30% EO cap) will introduce polyoxyethylene terminated molecules having more primary hydroxyl groups. This procedure, however, has several drawbacks. One of these drawbacks is that the large amount of polyoxyethylene content considerably alters important properties such as hydrophobicity and hygroscopicity and may confer often unwanted surfactant properties by establishing or altering hydrophile/lipophile balance.
Unlike basic catalysts, double metal cyanide (“DMC”) catalysts do not significantly promote the isomerization of propylene oxide. As a result, DMC catalysts can be used to prepare polyols which have low unsaturation values and relatively high molecular weights.
Surprisingly, I discovered that high hydroxyl equivalent molecular weight defoamers based on polyoxyalkylene copolymers prepared in the presence of a catalyst, preferably, a DMC catalyst, exhibit excellent defoaming properties at a low cloud point. Additionally, I discovered that high hydroxyl equivalent molecular weight defoamers based on polyoxyalkylene copolymers prepared in the presence of a catalyst, preferably, a DMC catalyst, exhibit defoaming properties which are better than those of defoamers having the same cloud point which are based on polyoxyalkylene copolymers prepared in the presence of KOH.